Auroras toss scientists surprises

Discoveries 06/15/98

The otherworldly glow of the northern lights fascinates scientists as much as tourists.

In places such as Alaska, the aurora borealis appears almost every clear night, in varying colors and shapes, from broad green draperies to reddish arcs and undulating bands. Not
surprisingly, visitors are always asking the University of Alaska's Geophysical Institute what causes such a magnificent celestial sight.

Unfortunately, scientists know the basics, but beyond that, they don't have much of a clue.

"The more you learn about the aurora, the less you seem to know," said Joseph
Borovsky, an aurora researcher at Los Alamos National Laboratory in New
Mexico.

"People in the 1960s and '70s had a very simple picture of how the aurora
worked," he added. "When they got satellites flying through, things got very
complicated."

Nowadays, researchers have a host of techniques to plumb auroras, from flying planes beneath the lights to peering down from above. With these new methods, physicists are
finding some surprises, such as the fact that auroras don't actually become more frequent during times when the sun is most active. Other recent work has suggested that auroras
surprisingly tend to follow coastlines, like a ship hugging the coast. And a new satellite is revealing in even more detail just how little scientists know about the complex atmospheric processes that create auroras.

For centuries, people have struggled to explain the aurora. Maybe it was sunlight bouncing off the snow and ice near the poles, they suggested, or light that was refracted like a rainbow. Or perhaps the aurora was light coming off the edges of the world.

Today, scientists know that an aurora results from the steady stream of charged particles that flow from the sun. This flow, known as the solar wind, collides with Earth's magnetic
field. Some of the electrons within the solar wind become channeled along the strong magnetic field lines, which curve into the planet's poles. As the electrons approach the polar regions, they speed up and finally slam into atoms and molecules in the upper atmosphere, releasing their energy as the auroral lights. In this sense, an aurora works something like a television set, which glows when beams of fast-moving electrons strike the screen.

In general, scientists think this basic picture is correct. But sometimes, the aurora tosses them a few surprises.

Auroras and the sun

Scientists once assumed that the aurora was always up in the sky, but just washed out by sunlight, said physicist Marc Hairston of the University of Texas at Dallas. (X-ray images
support the idea that an auroral oval can stretch into both day and night regions.) But in 1996, Johns Hopkins University researchers reported that the aurora occurs less frequently
in the daytime than at night.

Now, the Johns Hopkins team has found that auroras are less likely to occur when the sun is more active, which is exactly opposite of what scientists had thought.

The sun goes through an 11-year cycle of activity; at its peak, called solar maximum, the sun becomes mottled with more sunspots and sends out a stronger solar wind. Most scientists have thought that a stronger solar wind should create more
auroras. In fact, people have reported seeing the aurora at lower latitudes during solar maximum, even in Texas - and as far south as Mexico and the Caribbean, said Dr. Hairston.

But the Johns Hopkins researchers, from the Applied Physics Laboratory in Laurel, Md., found that fewer auroras actually occur during solar maximum.

The researchers looked at 12 years' worth of data from the Air Force's Defense Meteorological Satellite Program. At any time, at least two of these satellites are orbiting Earth, sending weather information to the military as well as information on electron streams to auroral scientists.

By looking at so much data, over an entire solar cycle, the Johns Hopkins team found that auroras occur no more than usual at night during solar maximum, and even fewer occur in daylight hours during that time. The scientists reported their work last month in the journal Nature.

The study confirms an earlier theory that describes how the sun's ultraviolet rays change how well the atmosphere conducts electricity.

"In retrospect, it seems like it should have made sense all
along," said Patrick Newell, leader of the team.

According to the earlier theory, more ultraviolet rays - as there are during solar maximum - increase the atmosphere's conductivity, leading to fewer auroras during sunlight hours.
Darkness - lacking the sun's UV rays - brings no similar effect, so the number of auroras remains the same at that time.

The next solar max is due around 2000.

Another recent study also seems to turn some accepted aurora knowledge on its head by suggesting that auroras tend to hug the coastline.

This phenomenon has been noted at least since 1820, when Russian explorer Ferdinand Von Wrangel wrote in his journal how the northern lights along the edge of Siberia seemed to wander along the shore.

Picture study

Now, University of Iowa physicists have pored over some 8,600 pictures taken by NASA's Polar satellite to see whether the folklore of "coastline auroras" might have some basis in fact.

Louis Frank and his colleagues found that in every hundred auroras, several would show a tendency to follow a coastline. In some cases, this meant that an auroral arc would bend
dramatically once it encountered land; in others, an arc would fade over land and reappear on the other side, over the sea.

Dr. Frank suggested that because electric currents move much more strongly within seawater than within land, that difference can actually affect the structure of the aurora, high in the atmosphere. According to this theory, the lights would be able to "sense" the difference in electrical conductivity between land and sea, in certain circumstances.

"The aurora is remarkable in itself, but for the ground to
somehow affect the aurora at very special times, that's
extremely interesting," Dr. Frank said last month in Boston
at a meeting of the American Geophysical Union.

But not every scientist agrees with this theory, and those who do point out that it is such a small effect that it's hard to tell whether its occurrence is statistically significant.

Tuija Pulkkinen, a researcher with the Finnish Meteorological Institute in Helsinki, has looked for similar coastline auroras along the Norwegian coastline. She and her team found that, rarely, the lights would appear to bend along the coastline as Dr. Frank's team saw. And the events lasted only about five to 10 minutes before the auroras changed shape and moved on.

"This is a secondary effect that can't really control the
existence or formation of auroras, but can control where and
how they appear," she said at the Boston meeting.

Satellite insights

Some other insights into auroras come from the Fast Auroral Snapshot Explorer satellite, nicknamed FAST.

This spacecraft, launched in August 1996, is sending back new data on the complex electrical and magnetic interactions required to form auroras, says its principal investigator, Charles Carlson of the University of California, Berkeley.

For example, FAST has found that electrons also zoom back out of the atmosphere, just as they zoom inward on the solar wind to create the aurora.

"There are huge fluxes of electrons coming out of [some]
regions," said Dr. Carlson. "They're as intense - or even
more intense - than the electrons coming in to make the
aurora."

In a way, this makes sense, because electrons would need to flow both up and down in the atmosphere in order to maintain an electric current, he added. It's just that the electrons moving up into the atmosphere don't betray their presence by giving off light, as the aurora does.

FAST has also shown scientists just how important the other charged particles in the solar wind are to the auroral process. Positively charged particles, or ions, also flow in and out of the atmosphere in huge amounts and in complex patterns, he said.

Only this spacecraft has provided such complete coverage of auroras in time and space, he added.

"In the past, satellites would be sweeping like
searchlights," Dr. Carlson said. "If you weren't looking in
the right direction at the right time, you wouldn't see
something."

Over the next few years, until solar maximum, scientists plan to keep using results from FAST, the Polar satellite, and experiments flown on rockets and jets to learn more about auroral processes.

"I think we've made most of the surprising discoveries as far
as the new data," said Dr. Carlson. "Now it's another big
step to understanding how it all fits together - and that's a
far lengthier process."
Images of the aurora, taken by Jan Curtis of the Geophysical
Institute in Fairbanks, are available at: www.geo.mtu.edu/weather/aurora/images/aurora/jan.curtis/
* The Applied Physics Laboratory has a page describing
auroral imaging, including the latest work on auroral
suppression during solar maximum.
* The University of Iowa site on coastline auroras.
* The University of California, Berkeley's page on the
science coming from the Fast Auroral Snapshot Explorer
satellite is at: plasma2.ssl.berkeley.edu/fast
* A site on exploring Earth's magnetosphere is at:
www-spof.gsfc.nasa.gov/istp/outreach